Petrology of a transform fault zone and adjacent ridge segments

1971 ◽  
Vol 268 (1192) ◽  
pp. 423-441 ◽  
Keyword(s):  
Oceanic Crust ◽  
Fault Zone ◽  
Fracture Zone ◽  
Volcanic Eruptions ◽  
Spreading Ridge ◽  
Transform Fault ◽  
Transform Faults ◽  
Near Surface ◽  
The North ◽  
Transform Fault Zone

The Verna Fracture Zone in the North Atlantic (9 to 11° N), which has been identified as a transform fault zone, contains exposures of serpentinized peridotites, while its adjacent ridge segments are floored mainly by typical abyssal ocean ridge basalts. This petrologic contrast correlates with the greater frequency of volcanic eruptions along the actively spreading ridge segments compared to the transform fault zone. Where rifting components occur across transform faults, exposures of the deeper zone of oceanic crust may result. The bathymetry of the Verna Fracture Zone suggests that some uplift parallel to the fracture zone as well as rifting led to exposures of deeper rocks. The basalts from the adjacent ridge axes contain ‘xenocrysts’ of plagioclase and olivine and more rarely of chromite. These appear to have a cognate origin, perhaps related to cooling and convection in near surface magma chambers. The basalts from the ridge axes, offset and on opposite sides of the transform fault, have similar features and compositions. The plagioclase peridotites have mineralogical features which indicate equilibration in the plagioclase pyrolite facies, suggesting maximum equilibration depths of around 30 km for a temperature of around 1200 °C. The chemical characteristics of the Vema F.Z. peridotites suggest that they may be undifferentiated mantle, emplaced as a subsolidus hot plastic intrusion or as a crystal mush. The abundance of peridotites and serpentinized peridotites is believed to reflect their abundance in seismic layer three of the oceanic crust.

scholarly journals On the Segmentation of the Cephalonia–Lefkada Transform Fault Zone (Greece) from an InSAR Multi-Mode Dataset of the Lefkada 2015 Sequence

2019 ◽  
Vol 11 (16) ◽  
pp. 1848
Author(s):  
Nikos Svigkas ◽  
Simone Atzori ◽  
Anastasia Kiratzi ◽  
Cristiano Tolomei ◽  
Andrea Antonioli ◽  
...  
Keyword(s):  
Fault Zone ◽  
Surface Displacement ◽  
Interferometric Synthetic Aperture Radar ◽  
Transform Fault ◽  
Earthquake Relocation ◽  
The North ◽  
Available Information ◽  
Multi Mode ◽  
Additional Constraints ◽  
Transform Fault Zone

We use Interferometric Synthetic Aperture Radar (InSAR) to study the Cephalonia–Lefkada Transform Fault Zone (CTF) in the Ionian Sea. The CTF separates continental subduction to the north from oceanic subduction to the south, along the Hellenic Subduction Zone. We exploit a rich multi-modal radar dataset of the most recent major earthquake in the region, the 17 November 2015 Mw 6.4 event, and present new surface displacement results that offer additional constraints on the fault segmentation of the area. Based on this dataset, and by exploiting available information of earthquake relocation, we propose a new rupture process for the 2015 sequence, complementary to those published already. Our modelling includes an additional southern fault segment, oblique to the segment related with the mainshock, which indicates that the CTF structure is more complex than previously believed.

Magma-rich transform margins: example from the Limpopo transform fault zone between Mozambique and Antarctica

2020 ◽  
Author(s):  
Sylvie Leroy ◽  
Vincent Roche ◽  
François Guillocheau ◽  
Pierre Dietrich ◽  
Sidonie Revillon ◽  
...  
Keyword(s):  
Fault Zone ◽  
Magnetic Data ◽  
Transform Fault ◽  
Passive Margins ◽  
The North ◽  
Oceanic Spreading ◽  
Transform Margin ◽  
Seismic Reflection Profiles ◽  
Uplift And Erosion ◽  
Transform Fault Zone

<p>Transform continental margins known across the Earth represent 31% of passive margins. Resulting from first-order plate tectonic processes, transform margins record a diachronous evolution mainly defined by three successive stages, including intra-continental transform faulting, active and passive transform margin. Due to their high complexity and a lack of large hydrocarbon discoveries (i.e. not a target for oil industry), they have only been sparsely studied, especially when compared with other margin types (i.e. divergent or convergent).</p><p>                  We present the structure and evolution of the NS-trending Limpopo Transform Fault Zone (LTFZ), corresponding to the main fracture zone from western part of the Africa-Antarctica Corridor (AAC). Here, we combine published and unpublished dataset (seismic reflection profiles, wells, multibeam bathymetry, gravity, magnetic data) in order to propose an interpretation of the LTFZ structure and adjoining segments and their evolution through time, from rifting to spreading.</p><p>The LTFZ is composed of two main segments: the East Limpopo segment and the Astrid conjugate one and the North and South Natal segment including the Dana-Galathea Plateau (Mozambique side) and the Maud rise/east of Grunehogna craton (Antarctica margin). The LTFZ offsets the segments of divergent conjugate margins (Southern Natal-off Grunehogna craton in the west and Beira High Angoche-Riiser Larsen Sea in the east) since 155 Ma (chron M25). We focus on the evolution of the transform fault zone from its initiation at chron M25 up to chron M0 (~126 Ma, Barremian). Oceanic spreading onset at chron M25 in the south of Beira High segment and Dana-Galathea Plateau triggered the uplift and erosion of the proximal parts of the margin and the formation of several seaward dipping reflectors wedges. Plate kinematic implies an NNW-SSE opening of the LTFZ. The oblique component of opening promotes the setting up of several volcanic wedges. These wedges rejuvenate southward trough time, which is consistent with the sliding of Antarctica with respect to Africa and thus confirm the diachronous evolution of the transform fault zone.</p>

scholarly journals Seismic investigation of an active ocean–continent transform margin: the interaction between the Swan Islands Fault Zone and the ultraslow-spreading Mid-Cayman Spreading Centre

2019 ◽  
Vol 219 (1) ◽  
pp. 159-184 ◽  
Author(s):  
C Peirce ◽  
A H Robinson ◽  
A M Campbell ◽  
M J Funnell ◽  
I Grevemeyer ◽  
...  
Keyword(s):  
Continental Crust ◽  
Oceanic Crust ◽  
Fault Zone ◽  
Lower Crust ◽  
P Wave ◽  
Fault System ◽  
Transform Fault ◽  
Apparent Lack ◽  
The North ◽  
Transform Margin

SUMMARY The Swan Islands Transform Fault (SITF) marks the southern boundary of the Cayman Trough and the ocean–continent transition of the North American–Caribbean Plate boundary offshore Honduras. The CAYSEIS experiment acquired a 180-km-long seismic refraction and gravity profile across this transform margin, ∼70 km to the west of the Mid-Cayman Spreading Centre (MCSC). This profile shows the crustal structure across a transform fault system that juxtaposes Mesozoic-age continental crust to the south against the ∼10-Myr-old ultraslow spread oceanic crust to the north. Ocean-bottom seismographs were deployed along-profile, and inverse and forward traveltime modelling, supported by gravity analysis, reveals ∼23-km-thick continental crust that has been thinned over a distance of ∼70 km to ∼10 km-thick at the SITF, juxtaposed against ∼4-km-thick oceanic crust. This thinning is primarily accommodated within the lower crust. Since Moho reflections are not widely observed, the 7.0 km s−1 velocity contour is used to define the Moho along-profile. The apparent lack of reflections to the north of the SITF suggests that the Moho is more likely a transition zone between crust and mantle. Where the profile traverses bathymetric highs in the off-axis oceanic crust, higher P-wave velocity is observed at shallow crustal depths. S-wave arrival modelling also reveals elevated velocities at shallow depths, except for crust adjacent to the SITF that would have occupied the inside corner high of the ridge-transform intersection when on axis. We use a Vp/Vs ratio of 1.9 to mark where lithologies of the lower crust and uppermost mantle may be exhumed, and also to locate the upper-to-lower crustal transition, identify relict oceanic core complexes and regions of magmatically formed crust. An elevated Vp/Vs ratio suggests not only that serpentinized peridotite may be exposed at the seafloor in places, but also that seawater has been able to flow deep into the crust and upper mantle over 20–30-km-wide regions which may explain the lack of a distinct Moho. The SITF has higher velocities at shallower depths than observed in the oceanic crust to the north and, at the seabed, it is a relatively wide feature. However, the velocity–depth model subseabed suggests a fault zone no wider than ∼5–10 km, that is mirrored by a narrow seabed depression ∼7500 m deep. Gravity modelling shows that the SITF is also underlain, at >2 km subseabed, by a ∼20-km-wide region of density >3000 kg m−3 that may reflect a broad region of metamorphism. The residual mantle Bouguer anomaly across the survey region, when compared with the bathymetry, suggests that the transform may also have a component of left-lateral trans-tensional displacement that accounts for its apparently broad seabed appearance, and that the focus of magma supply may currently be displaced to the north of the MCSC segment centre. Our results suggest that Swan Islands margin development caused thinning of the adjacent continental crust, and that the adjacent oceanic crust formed in a cool ridge setting, either as a result of reduced mantle upwelling and/or due to fracture enhanced fluid flow.

scholarly journals The 2014 Kefalonia Doublet (MW6.1 and MW6.0), Central Ionian Islands, Greece: Seismotectonic Implications along the Kefalonia Transform Fault Zone

2015 ◽  
Vol 63 (1) ◽  
pp. 1-16 ◽  
Author(s):  
Vassilios Karakostas ◽  
Eleftheria Papadimitriou ◽  
Maria Mesimeri ◽  
Charikleia Gkarlaouni ◽  
Parthena Paradisopoulou
Keyword(s):  
Fault Zone ◽  
Transform Fault ◽  
Ionian Islands ◽  
Transform Fault Zone

Evolution of migrating transform faults in anisotropic oceanic crust: examples from Iceland

2019 ◽  
Vol 56 (12) ◽  
pp. 1297-1308 ◽  
Author(s):  
Jeffrey A. Karson ◽  
Bryndís Brandsdóttir ◽  
Páll Einarsson ◽  
Kristján Sæmundsson ◽  
James A. Farrell ◽  
...  
Keyword(s):  
Plate Boundary ◽  
Fault Zones ◽  
Plate Boundaries ◽  
Transform Fault ◽  
Eurasian Plate ◽  
Transform Faults ◽  
The North ◽  
Rift Propagation ◽  
Oriented Parallel ◽  
Ocean Ridge

Major transform fault zones link extensional segments of the North American – Eurasian plate boundary as it transects the Iceland Hotspot. Changes in plate boundary geometry, involving ridge jumps, rift propagation, and related transform fault zone migration, have occurred as the boundary has moved relative to the hotspot. Reconfiguration of transform fault zones occurred at about 6 Ma in northern Iceland and began about 3 Ma in southern Iceland. These systems show a range of different types of transform fault zones, ranging from diffuse, oblique rift zones to narrower, well-defined, transform faults oriented parallel to current plate motions. Crustal deformation structures correlate with the inferred duration and magnitude of strike-slip displacements. Collectively, the different expressions of transform zones may represent different stages of development in an evolutionary sequence that may be relevant for understanding the tectonic history of plate boundaries in Iceland as well as the structure of transform fault zones on more typical parts of the mid-ocean ridge system.

scholarly journals New observational insights into the atmospheric circulation over the Euro-Atlantic sector since 1685

2019 ◽  
Vol 54 (1-2) ◽  
pp. 823-841 ◽  
Author(s):  
Javier Mellado-Cano ◽  
David Barriopedro ◽  
Ricardo García-Herrera ◽  
Ricardo M. Trigo
Keyword(s):  
Atmospheric Circulation ◽  
Wind Direction ◽  
Volcanic Eruptions ◽  
Atlantic Sector ◽  
Near Surface ◽  
The North ◽  
External Forcings ◽  
Cardinal Direction ◽  
Daily Scale ◽  
The North Atlantic Oscillation

Abstract Wind direction kept in ships’ logbooks is a consolidated but underexploited observational source of relevant climatic information. In this paper, we present four indices of the monthly frequency of wind direction, one for each cardinal direction: Northerly (NI), Easterly (EI), Southerly (SI) and Westerly (WI), based on daily wind direction observations taken aboard ships over the English Channel. These Directional Indices (DIs) are the longest observational record of atmospheric circulation to date at the daily scale, covering the 1685–2014 period. DIs anomalies are associated with near-surface climatic signals over large areas of Europe in all seasons, with zonal indices (WI and EI) and meridional indices (NI and SI) often affecting different regions. Statistical models including all DIs are able to explain a considerable amount of European climate variability, in most cases higher than that accounted for by the North Atlantic Oscillation. As such, the DIs are able to reproduce the known European climatic history and provide new insights of certain episodes from monthly to multi-decadal time scales such as the warm winter decade of 1730–1739 or the extremely cold 1902 summer. The DIs show the potential to better constrain the atmospheric circulation response to external forcings and its associated anomalies. In particular, we provide first observational evidences of all year-round atmospheric circulation signals following the strongest tropical volcanic eruptions of the last three centuries. These signatures are more complex than previously thought and suggest that the well-reported winter warming and summer cooling cannot be simply interpreted in terms of changes in zonality.

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